We intend further to define the new model of lower intestinal cancer induced by polycyclic hydrocarbons (especially methylcholanthrene (MC)) in inbred Syrian hamsters (Homburger et al., Cancer Res. 32:360-366, 1972) especially with regards to the following factors: sex (castration of males and females, reciprocal estrogen and androgen substitution); biochemistry of carcinogen by studying the induced liver benzo(a)pyrene hydroxylases in susceptible and resistant animals and by following excretion and metabolism of labeled MC in feces and various organs; immunosuppression achieved by rabbit anti-hamster antilymphocytic serum; genetics studied by cross-breeding the most susceptible and most resistant strains and studying the offspring and subsequent generation; and to obtain serially transplantable hamster tumors of the small intestine grown in the hamster cheek pouch. This project is responsive to several questions stated in the announcement of the National Large Bowel Cancer Project of October 6, 1972, such as cell-mediated immunity, carcinogenesis of large bowel cancer in experimental animals, genetics of large bowel cancer.